In recent years, the needs for laser beam machines, which are used for micro machining such as printed-circuit-board machining, for generating pulses whose outputted width is approximately from 1 μs to several dozen μs, have increased, and the machines have been put into practical use.
A basic configuration of a conventional pulse laser generator used in a gas laser beam machine (hereinafter referred to as a pulse laser generator) is illustrated in FIG. 9.
The pulse laser generator is configured in such a way that an electric power supplying unit 3 (composed of, for example, a three-phase rectifier circuit 4, an inverter circuit 5, and a step-up transformer 6) is controlled by command pulse sets 2 outputted from a controller 1, so that electric discharge occurs by electric power being supplied to a discharging space 7 filled with a laser medium (mixed gas), the laser medium pumped by the discharge creates a laser beam 12 in a resonator 8 (composed of electrodes 9, a partially reflecting mirror 10, and a fully reflecting mirror 11), and the laser beam is outputted.
Specifically, when the command pulse sets 2 are outputted from the controller 1, the inverter circuit 5 correspondingly operates and transforms into ac electric power dc electric power rectified in the three-phase rectifier circuit 4, then the ac electric power is stepped-up to a required voltage for the discharge by the step-up transformers.
Here, in the electric power unit used in an industrial gas-laser-beam machine (such as a carbon dioxide gas laser machine), the ac electric power supplied to generate the discharge for pumping the laser medium is, generally, that an applied voltage to the electrodes (hereinafter referred to as a discharging voltage) is several kV, flowing current during discharging (hereinafter referred to as discharging current) is ten and several A (ampere) at its peak, and an ac frequency (hereinafter referred to as a discharging frequency) during discharging is equal to or more than several hundred kHz; consequently, in a case of a pulse laser generator, corresponding to the command pulse sets 2 from the controller 1, the ac power is supplied to the discharging space (the number of the ac components is equal to a switching number N of the inverter circuit 5), and a laser beam is resultantly outputted as illustrated in FIG. 10.
Here, the electric power such as this for discharging is defined to be electric discharging power in this specification.
Then, the machining is performed using each (pulse) of the laser pulses outputted, as described above, being irradiated onto a workpiece to be processed.
Next, the controller 1 for outputting the command pulse sets 2 will be explained in detail.
In the pulse laser beam machines used for micro machining such as printed circuit board machining, principal control parameters have been set for controlling the laser output power of the pulse laser generator, which are, for example, peak output power representing a peak power value of the laser pulses outputted, a repeated pulse frequency representing a frequency for the laser pulses outputted, and a pulse width representing a pulse width of the laser pulses outputted. In addition, the most suitable value of energy per pulse of the laser output (hereinafter referred to as pulse energy) represented by “peak output×pulse width” has been obtained depending on workpiece materials to be machined and machining methods, and each value of the above described control parameters is correspondingly determined.
Here, these control parameters are arbitrarily settable, and the command pulse sets 2 are outputted in response to the parameters that have been set.
The machine system is designed in such a way that the command pulse sets 2 in the controller 1 using these parameters are outputted, so that the laser pulse output power required for the machining is obtained.
In the electric power supplying unit 3, in order to obtain laser pulse energy required for the machining, the electric discharging power peak for controlling the peak output power is controlled using the command pulse sets 2 that are outputted based on the controlling parameters set in the controller 1, and the repeated pulse frequency is controlled; thus, increasing or decreasing of the number of switching times (hereinafter referred to as the switching number) of the inverter circuit 5 is controlled so that the pulse width is controlled.
In addition, because the electric discharging power is principally determined by the discharging voltage applied to the electrodes 9 and the peak of the discharging current that flows caused by the discharging between the electrodes, in order to control the electric discharging power peak, a method is adopted in which the discharging voltage is controlled, or the discharging current peak is controlled, for example, by the inverter circuit 5 being controlled using a PWM (pulse width modulation) controller.
The repeated pulse frequency is equal to the number of the laser pulses irradiated per second, and the number of the command pulse sets, which are outputted from the controller per second.
Here, the repeated pulse frequency is generally much lower than the above described discharging frequency (that is, the switching frequency of the inverter circuit 5); for example, the repeated pulse frequency is at most several kHz while the discharging frequency as described above is higher than several hundred kHz.
The pulse width is determined, corresponding to each of the command pulse sets, by the pulse number of the pulse sets of the electric discharging power that is outputted through the inverter circuit. For example, with respect to the laser pulse output power having a pulse width t as represented in FIG. 10, in a case in which the pulse width needs to be expanded to double (2 t), the laser pulse width becomes double (2 t) by the switching number N of the inverter circuit being set at double (2N) as represented in FIG. 12.
Next, a relationship between the peak output power and the pulse width of the pulse laser outputted from the pulse laser generator is illustrated in FIG. 13.
Here, an area described by “peak output power x pulse width” represents pulse energy.
The upper limit of the pulse energy is determined by the light resistance specification of the fully reflecting mirror and partially reflecting mirror that configure the resonator unit of the laser generator (FIG. 9).
Therefore, for example, in a case in which the pulse energy (=p1×t1) with peak output power of p1 and pulse width of t1 is in the upper limit value of the energy based on the light resistance specification of the mirrors, when the pulse width is required to be expanded from t1 to t2 (t1<t2), if the pulse width is expanded from t1 to t2 with the peak output power p1 being constant, the energy (=p1×t1) exceeds the upper limit of the light resistance of the mirrors, which can cause the mirrors burning out; therefore, the pulse width cannot be simply expanded, and, in such a case, the peak output power needs to be decreased from p1 to p2 (p1×t1≧p2×t2).
In a case in which the peak output power is varied in such a manner as this, although a method in which the discharging voltage applied to the electrodes is varied is generally used, the larger the discharging voltage or the discharging current becomes with respect to the rating of the electric power supplying unit, the larger the load becomes with respect to the electric power supplying unit, meanwhile, the smaller the discharging voltage becomes, the more unstable the discharging becomes (discharge generation becomes difficult); therefore, the variable width of the voltage applied is in general approximately ten percent of the rated voltage.
Now, materials and kinds of machining have been diversified in the micro machining using laser beam machines; that is, because there are cases in which the shorter the laser irradiation time (that is, the narrower the pulse width), the higher the machining quality can be obtained, for example, in a case of machining polyimide resin, and the relatively longer the laser irradiation time (that is, the wider the pulse width), the higher the quality machining can be obtained, for example, in a case of machining glass/epoxy materials containing glass fibers, a pulse laser beam machine in which its pulse width can be considerably varied is demanded.
However, in the conventional control method for the electric power supplying unit used in the gas laser beam machine, because the variable width of the voltage applied is relatively narrow, the pulse width variations must be limited in order to effectively generate the electric discharging without exceeding the upper limit of the light resistance of the mirrors; therefore, it has been difficult to considerably vary the pulse width (for example, varied from 1 μs or shorter than 1 μs to several hundred μs).
Here, it is obviously ineffective from point of its cost vs. performance to use mirrors having higher upper limits of the light resistance.
Moreover, regarding the pulse width control, provided that the switching number of the inverter circuit 5 in the electric power supplying unit 3 is N, the pulse width t of the laser output power is expressed byt∝Nas understood also from FIG. 10 and FIG. 12; therefore, in order to increase the pulse width the switching number N needs to be increased.
However, if the switching number of times N increases, the switching loss in a semiconductor device used in the electric power supplying unit increases in proportional to the switching number; consequently, a problem may occur in which the heat generated by the electric power supplying unit increases.
In such a case, by, for example, facilitating more cooling mechanisms in the electric power supplying unit or increasing the number of the devices or circuits installed in parallel, the capacity of the electric power supplying unit itself needs to be increased; as a result, it entails significant size increase of the apparatus itself, bringing disadvantages with respect to not only the cost but also the space for installing the apparatus.